Conventionally, in such a wind turbine generator for performing interconnected operation together with a utility grid, by controlling the rotational speed of a rotor and excitation current of the rotor, active power and reactive power are controlled, and the active and reactive powers are supplied to the utility grid.
Patent citation 1 discloses a technique for detecting a frequency of a utility grid, determining an active power demand value so that a grid frequency is equal to a predetermined value, and controlling the active power based on the active power demand value.    Patent Citation 1: Japanese Unexamined Patent Application, Publication No. 2002-44867